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Versions: 00 01 02 03 RFC 2539

INTERNET-DRAFT                            Diffie-Hellman Keys in the DNS
                                                              March 1998
                                                  Expires September 1998




     Storage of Diffie-Hellman Keys in the Domain Name System (DNS)
     ------- -- -------------- ---- -- --- ------ ---- ------ -----

                         Donald E. Eastlake 3rd



Status of This Document

   This draft, file name draft-ietf-dnssec-dhk-02.txt, is intended to be
   become a Proposed Standard RFC.  Distribution of this document is
   unlimited. Comments should be sent to the DNS security mailing list
   <dns-security@tis.com> or to the author.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months.  Internet-Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet-
   Drafts as reference material or to cite them other than as a
   ``working draft'' or ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (East USA), ftp.isi.edu (West USA),
   ftp.nordu.net (North Europe), ftp.nis.garr.it (South Europe),
   munnari.oz.au (Pacific Rim), or ftp.is.co.za (Africa).


















Donald E. Eastlake 3rd                                          [Page 1]


INTERNET-DRAFT                            Diffie-Hellman Keys in the DNS


Abstract

   A standard method for storing Diffie-Hellman keys in the Domain Name
   System is described which utilizes DNS KEY resource records.



Acknowledgements

   Part of the format for Diffie-Hellman keys and the description
   thereof was taken from an Internet draft by:

        Ashar Aziz <ashar.aziz@eng.sun.com>
        Tom Markson <markson@incog.com>
        Hemma Prafullchandra <hemma@eng.sun.com>

   In addition, the following person provided useful comments that have
   been incorporated:

        Ran Atkinson <rja@inet.org>
































Donald E. Eastlake 3rd                                          [Page 2]


INTERNET-DRAFT                            Diffie-Hellman Keys in the DNS


Table of Contents

      Status of This Document....................................1

      Abstract...................................................2
      Acknowledgements...........................................2

      Table of Contents..........................................3

      1. Introduction............................................4

      2. Diffie-Hellman KEY Resource Records.....................5

      3. Performance Considerations..............................6
      4. Security Considerations.................................6

      References.................................................7
      Author's Address...........................................7
      Expiration and File Name...................................7
































Donald E. Eastlake 3rd                                          [Page 3]


INTERNET-DRAFT                            Diffie-Hellman Keys in the DNS


1. Introduction

   The Domain Name System (DNS) is the current global hierarchical
   replicated distributed database system for Internet addressing, mail
   proxy, and similar information. The DNS has been extended to include
   digital signatures and cryptographic keys as described in [draft-
   ietf-dnssec-secext2-*.txt].  Thus the DNS can now be used for secure
   key distribution.

   This document describes how to store Diffie-Hellman keys in the DNS.
   Familiarity with the Diffie-Hellman key exchange algorithm is assumed
   [Schneier].

   Diffie-Hellman requires two parties to interact to derive keying
   information which can then be used for authentication.  Since DNS SIG
   RRs are primarily used as stored authenticators of zone information
   for many different resolvers, no Diffie-Hellman algorithm SIG RR is
   defined. For example, assume that two parties have local secrets "i"
   and "j".  Assume they each respectively calculate X and Y as follows:

        X = g**i ( mod p )
        Y = g**j ( mod p )

   They exchange these quantities and then each calculates a Z as
   follows:

        Zi = Y**i ( mod p )
        Zj = X**j ( mod p )

   Zi and Zj will both be equal to g**(ij)(mod p) and will be a shared
   secret between the two parties that an adversary who does not know i
   or j will not be able to learn from the exchanged messages (unless
   the adversary can derive i or j by performing a discrete logarithm
   mod p which is hard for strong p and g).

   The private key for each party is their secret i (or j).  The public
   key is the pair p and g which must be the same for the parties and
   their individual X (or Y).














Donald E. Eastlake 3rd                                          [Page 4]


INTERNET-DRAFT                            Diffie-Hellman Keys in the DNS


2. Diffie-Hellman KEY Resource Records

   Diffie-Hellman keys are stored in the DNS as KEY RRs using algorithm
   number 2.  The structure of the RDATA portion of this RR is as shown
   below.  The first 4 octets, including the flags, protocol, and
   algorithm fields are common to all KEY RRs as described in [draft-
   ietf-dnssec-secext2-*.txt].  The remainder, from prime length through
   public value is the "public key" part of the KEY RR. The period of
   key validity is not in the KEY RR but is indicated by the SIG RR(s)
   which signs and authenticates the KEY RR(s) at that domain name.

                            1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           KEY flags           |    protocol   |  algorithm=2  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     prime length (or flag)    |  prime (p) (or special)       /
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       /  prime (p)  (variable length) |       generator length        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | generator (g) (variable length)                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     public value length       | public value (variable length)/
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       /  public value (g^i mod p)    (variable length)                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Prime length is length of the Diffie-Hellman prime (p) in bytes if it
   is 16 or greater.  Prime contains the binary representation of the
   Diffie-Hellman prime with most significant byte first (i.e., in
   network order). If "prime length" field is 1 or 2, then the "prime"
   field is actually an unsigned index into a table of up to 65,536
   predefined prime/generator pairs to be defined in which case the
   generator length should be zero.  The meaning of a zero or 3 through
   15 value for "prime length" is reserved.

   Generator length is the length of the generator (g) in bytes.
   Generator is the binary representation of generator with most
   significant byte first.  PublicValueLen is the Length of the Public
   Value (g**i (mod p)) in bytes.  PublicValue is the binary
   representation of the DH public value with most significant byte
   first.

   The corresponding algorithm=2 SIG resource record is not used so no
   format for it is defined.







Donald E. Eastlake 3rd                                          [Page 5]


INTERNET-DRAFT                            Diffie-Hellman Keys in the DNS


3. Performance Considerations

   Current DNS implementations are optimized for small transfers,
   typically less than 512 bytes including overhead.  While larger
   transfers will perform correctly and work is underway to make larger
   transfers more efficient, it is still advisable at this time to make
   reasonable efforts to minimize the size of KEY RR sets stored within
   the DNS consistent with adequate security.  Keep in mind that in a
   secure zone, an authenticating SIG RR will also be returned.



4. Security Considerations

   Many of the general security consideration in [draft-ietf-dnssec-
   secext2-*] apply.  Keys retrieved from the DNS should not be trusted
   unless (1) they have been securely obtained from a secure resolver or
   independently verified by the user and (2) this secure resolver and
   secure obtainment or independent verification conform to security
   policies acceptable to the user.  As with all cryptographic
   algorithms, evaluating the necessary strength of the key is essential
   and dependent on local policy.

   In addition, the usual Diffie-Hellman key strength considerations
   apply. (p-1)/2 should also be prime, g should be primitive mod p, p
   should be "large", etc.  [Schneier]


























Donald E. Eastlake 3rd                                          [Page 6]


INTERNET-DRAFT                            Diffie-Hellman Keys in the DNS


References

   [RFC 1034] - P. Mockapetris, "Domain names - concepts and
   facilities", 11/01/1987.

   [RFC 1035] - P. Mockapetris, "Domain names - implementation and
   specification", 11/01/1987.

   [draft-ietf-dnssec-secext2-*.txt] - Domain Name System Security
   Extensions, D. Eastlake.

   [Schneier] - Bruce Schneier, "Applied Cryptography: Protocols,
   Algorithms, and Source Code in C", 1996, John Wiley and Sons




Author's Address

   Donald E. Eastlake 3rd
   CyberCash, Inc.
   318 Acton Street
   Carlisle, MA 01741 USA

   Telephone:   +1 978 287 4877
                +1 703 620-4200 (main office, Reston, VA)
   FAX:         +1 978 371 7148
   EMail:       dee@cybercash.com



Expiration and File Name

   This draft expires in September 1998.

   Its file name is draft-ietf-dnssec-dhk-02.txt.
















Donald E. Eastlake 3rd                                          [Page 7]


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